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A Systematic Method for Analyzing Borehole Recordings to Estimate the Wavefield in the Lakebed Zone of Mexico City

Masahiro IIDA
Earthquake Research Institute
University of Tokyo
1-1-1, Yayoi, Bunkyo-ku
Tokyo 113-0032, Japan

During the 1985 Michoacan earthquake (M_S = 8.1), large-amplitude seismograms with extremely long duration (hundreds of seconds) were recorded in the lakebed zone of Mexico City. In spite of numerous studies, the lakebed seismograms are not well understood. In the present study, we develop a systematic method for analyzing borehole recordings and apply it to recordings at depths of 0 and 102 m at the Roma station to interpret the wavefield in the lakebed zone. The target frequency (period) range is between 0.2 and 3.3 Hz (0.3 and 5.0 sec). We cross-correlate the surface and borehole strong-motion seismograms and identify body- and surface-wave portions in the main motions. Then, we interpret the surface waves as fundamental-mode Love waves by calculating theoretical amplitude ratios for different-scale structures. Using a statistical time-series model with an information criterion, we separate S- and Love-wave portions from the surface recordings. Love waves are dominant, and are recognized even in the early time section. Finally, we attempt to explain S-wave transfer functions between the surface and borehole recordings by considering subsidence of soft surficial deposits for several years on the basis of a three-dimensional finite-element technique. This simulation demonstrates effects of the subsidence on amplification of S waves.

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